This invention pertains to a method for production of storage-stable pellets from clarified sludge. In this method, the sludge is mechanically dewatered in a centrifuge, subsequently thermally dried with hot gas in the centrifuge housing, and shaped into pellets outside of the centrifuge. In addition, this invention pertains to a system to implement the method.
The clarified sludge occurring in large quantities in effluent clarification systems must be treated before its usage in agriculture, either by incineration, disposal in landfills which are increasingly in short supply or disposal, namely by means of mechanical, usually centrifugal, dewatering, thermal drying and shaping of the clarified sludge into agglomerates such as granulates or pellets, which promote or simplify the transport, handling and storage of the clarified sludge product.
In order to have a marketable use for the clarified sludge treated in this manner, in addition to a general absence of dust, the clarified sludge product must also be hygienically unremarkable, i.e., it must be essentially free of hazardous disease agents, and also a long-term shelf life of the product must be assured. Such a long-term shelf life, which in this case means the avoidance of biological reactions such as mold formation, can be achieved by thorough drying to a content of dry substance (TS) of over 92% with corresponding reduction in the residual moisture content. Even when utilizing the clarified sludge merely by incineration, often the end-user will demand a hygienically unremarkable product for safety reasons, in order to prevent, in absolute terms, any infection of operating personnel with potentially present disease bacteria.
Where a sanitizing of the clarified sludge cannot be definitely achieved in the treatment steps preceding the drying, such as by digestion of the sludge, then the sanitizing must take place by thermal treatment in the course of the drying, or in an additional post-treatment step.
In this regard various institutes and state agencies have projects which are aimed at obtaining a dependable sanitizing of clarified sludge by means of adherence to slightly differing time/temperature relations of the thermal treatment. For example, the requirements of the United States Environmental Protection Agency (abbreviated EPA) are internationally recognized; for pasteurization the EPA requirements call for a thermal treatment of pumpable clarified sludge of at least 30 min. at a temperature of at least 180xc2x0 C., or in the case of solid materials such as dried clarified sludge, a temperature of at least 70xc2x0 C. is required for a period of at least 30 min. In various known methods for thermal drying of clarified sludge, the EPA requirements are not satisfied, insufficiently met, or satisfied only at a relatively great expense for equipment. For example, in European Patent Document EP-B-0 396 852 it is disclosed how to dewater wet clarified sludge in a centrifuge, then to dry it in a contact dryer or pit dryer up to a content of dry substance TS of about 50% and then to shape the predried sludge in a device into sausage-shaped agglomerates, which are then dried to a TS-content of 90% on a belt dryer with flowing hot gas. However, due to unavoidable, irregular input of the clarified sludge onto the belt, local agglomerate piles can form which prevent a thorough drying and also a consistent heating. On the other hand, it is a known effect of this kind of irregular distribution of the clarified sludge on the belt that there are regions with greater material height and regions of lower material height, so that the hot air flow and thus the efficiency of the thermal energy is irregular when viewed across the surface of the belt. Thus, it is no longer possible to control the actual drying processes and an achieved dwell time of the clarified sludge product above the required temperature.
Also known is the combined centrifugal dewatering and thermal drying of clarified sludge in a compact structural unit in the form of a full jacket worm-gear centrifuge (German Patent Document DE-A-198 51 612), which is used as a dispersion element for a directly outlet-connected convection dryer operated with hot air or drying gas, in such a manner that the solids outlet housing, as well as the entire remaining housing of the centrifuge, is exposed to the flowing drying gas and is used as a flow dryer. Thus, the centrifugally dewatered thick material disgorged from the thick material outlet openings of the centrifuge drum is dispersed in swirling, spiral paths by the drying gas, dried to a dry substance content TS of about 70%, for example, and sent off by the drying gas outside of the centrifuge into a cyclone. Due to its residual moisture of about 30%, the clarified sludge can be shaped in a pelleting press without the use of a binder, to form relatively shape-stable sludge pellets. However, they do not satisfy the conditions stated above for being hygienically unremarkable, merely because the dwell time of the clarified sludge in the flow-drying portion of the centrifuge is very short. Of course, with this method it is also possible to achieve a clarified sludge product with a residual moisture content of less than 10%; however, the requirements stated above are not satisfied, due to the short dwell time of the clarified sludge product in the flow-dryer portion. A dwell time on the order of 30 min. and more is virtually impossible for this method. According to experience, no pellets can be produced from thoroughly dried, fine particulate, practically dust-like solid material using conventional methods, as numerous tests have shown. The addition of binders is prohibited for financial reasons, and under some circumstances will restrict the useful value of the dried clarified sludge.
Thus the situation arises that the method according to German Patent Document DE-A-198 51 612 would permit the production of a shelf-stable product, but which cannot be pressed into pellets, and pellet formation is a necessary condition for making the clarified sludge product essentially dust-free. On the other hand, the production of clarified sludge pellets is possible, but they will not have a long shelf life due to excessive residual moisture. Provided the clarified sludge has not been sanitized by a pretreatment step, such as by digestion, then the clarified sludge product can by no means be viewed as hygienically satisfactory.
The present invention is based on the problem of producing pellets or agglomerates in an economically feasible manner from clarified sludge which is dewatered in a full jacket worm-gear centrifuge and is subsequently thermally dried in the centrifuge housing, and which satisfies the conditions of being hygienically unremarkable and also storage-stable.
In a method according to the present invention, the pellets or agglomerates which are coming from the pelleting device outlet-connected to the centrifuge dryer, are not cooled down, but rather they are held quite the contrary, at their temperature of at least about 50xc2x0 C. in an enclosed sanitizing apparatus at the outlet from the pelleting device, by usage of the residual pellet heat for a long time of at least about 10 min, so that the clarified sludge pellets exiting from the sanitizing apparatus after passage of this dwell time, have become hygienically unremarkable. If the hot or warm pellets come from the pelleting press at a temperature of about 80xc2x0 C., for example, then the pellets need be heated very little or not at all, during their dwell time in the sanitizing apparatus.
At the same time, by keeping the clarified sludge pellets at the temperature level stated above, additional post-drying of the clarified sludge pellets will occur, so that the pellets will become harder and thus more stable in storage. Thus, the danger of mold formation on the clarified sludge pellets treated according to this invention will be reduced to zero, because the residual moisture of these pellets also goes to zero.
Only when the minimum temperature of the clarified sludge pellets in the sanitizing apparatus cannot be maintained for the entire dwell time is the use of additional energy required, e.g., the use of a hot gas stream in the sanitizing apparatus.
According to one special property of the invention, the clarified sludge pellets still hot or warm due to the thermal clarified sludge drying, are held in the sanitizing apparatus at a temperature of at least 70xc2x0 C. for a dwell time of at least 30 min. If the sanitizing apparatus has a hot gas flowing through it, then the additional advantages are attained that due to a suitable selection of volume flow and temperature of the hot gas, an excessively fast drying and break-up of the pellets will be avoided, and at the same time the fines of the pellets to be sanitized can be removed with the hot gas stream from the sanitizing apparatus.
If hot gas is needed in the sanitizing apparatus, then exhaust gas should be used which is provided by the drying itself, according to the method of German Patent Document DE-A-198 51 612, that is, an otherwise nonutilized exhaust gas which typically leaves the drying system at a temperature of about 120 to about 210xc2x0 C. After passing through the sanitizing apparatus, this exhaust gas moves into the (otherwise required) exhaust gas scrubbing system of the drying process. The sanitizing proposed by the invention and the attainment of a shelf life for the pellets of dried clarified sludge according to this invention is an integral constituent of the clarified sludge drying, so that no additional energy is needed and no additional exhaust gas is created, which under some circumstances would require additional post-treatment.